Generally speaking, in a fuel tank that is not completely full, on an interface between the fuel and the hollow space above it, which hollow space comprises air, there is a danger that at a particular temperature and at a corresponding pressure a potentially explosive mixture of air and fuel gases forms. As a result of a spark or some other source of ignition this mixture can be ignited and can result in an explosion of the fuel tank. In particular when the fuel tank is that of an aircraft, this has a catastrophic effect and could result in the loss of the aircraft including the crew and the passengers.
For this reason, for example, the FAA has introduced directives that prescribe a reduction in the explosion risk of an aircraft. A known solution to this problem consists of introducing a protective gas into the fuel tank of an aircraft, wherein the protective gas is oxygen depleted or fully inerted. This results in an air-fuel mixture that arises in the fuel tank being non-flammable because of the lack of oxygen. Thus from DE 10 2005 054885 A1 and US 2007/0111060 A1 a safety system for reducing the explosion risk of a fuel tank is known, in which system oxygen-depleted air emanates from an already installed fuel cell in the aircraft and is fed to the fuel tank. However, this is associated with a problem in that the protective gas comprises very high relative atmospheric humidity which has a negative effect on the quality of the fuel. In order to dehumidify the oxygen-depleted air, condensation devices are necessary. The water obtained from the protective gas can later be reused for other purposes. As a result of the condensation devices of the safety system the total weight of the aircraft is considerably increased. Furthermore, when the aircraft is on the ground, kerosene vapours are emitted from the ventilation system of the fuel tanks to the atmosphere.